Medicinal aerosol inhalers are commonly used to treat a number of medical conditions. Leakage and moisture ingress are two common problems that can limit the shelf life of a medicinal aerosol. Leakage of propellant from a metered dose inhaler (MDI) during storage will lead to an increase in concentration of the other components, such as drug and excipient, of the aerosol formulation. Since an MDI contains a fixed volume of formulation, changes in the formulation concentration can lead to a change in the delivery dosage of drug and excipient. This is a particular concern for products with low total fill weights, as small absolute amounts of leakage can represent a significant percentage of the total fill weight.
Thus, all propellant based MDIs use some system of seals and/or diaphragms to minimize or prevent leakage. In a conventional MDI there are two main places that must be sealed to prevent excessive leakage of the formulation, although depending on the particular canister design the number of areas to be sealed may vary. Typically a seal is made from an elastomer which forms a vapor-tight seal between two metal or plastic parts. One place to be sealed is the contact area between the canister and the ferrule. This is done with a static seal, in that no moving parts are involved once this seal is formed. Another place to seal is to allow for isolation of the metering valve from the outside of the can, which is done using a diaphragm seal. The diaphragm seal is a dynamic (i.e., reciprocating or sliding) seal, since the valve has to move in relation to the diaphragm seal in order to perform the metering function.
Numerous materials are well known in the art for use as seals in medicinal aerosols including butyl rubber, butadiene-acrylonitrile rubbers or “Buna”, neoprene or polychloroisoprene, ethylene-butene, ethylene-octene, ethylene-hexene, such as in U.S. Pat. No. 5,290,539 (Marecki) and EPDM, such as in U.S. Pat. No. 5,836,299, (Kwon). It is also recognized that these materials may be blended with one or more other polymers to achieve properties representative of the individual materials, such as in U.S. Pat. No. 5,474,758 (Kwon). Likewise, alternate configurations have been proposed to reduce leakage from medicinal aerosols, such as inclusion of an additional seal to provide a dual seal configuration for sealing the canister and the ferrule, such as in U.S. Pat. No. 5,775,321 (Alband) and U.S. Pat. No. 6,129,247 (Thomas et al.).